The influence of heat-treatment on regulating the content and morphology of LPSO phase in Mg-Y-Al alloy and its strengthening mechanism at room temperature

Abstract

The LPSO phase can effectively enhance the mechanical properties of Mg alloys. To investigate the impact of different LPSO phase contents and morphologies on the mechanical properties and strengthening mechanisms of Mg-Y-Al alloys under room temperature deformation, this study prepared Mg-12Y-1Al (WA121) alloys containing Bulk-LPSO (B-LPSO), Lattice-LPSO (L-LPSO), and Needle-like LPSO (N-LPSO) with different contents through different heat-treatment processes. The results indicate that with the increase in heat treatment time, the contents of B-LPSO phases remain essentially unchanged, and the contents of L-LPSO and N-LPSO phases gradually increase. The increase in N-LPSO phase content is the most pronounced, with the highest content (7.29%) observed in the alloy treated for 4.5 h. Moreover, the alloy treated for 4.5 h exhibits the best mechanical properties, with ultimate tensile strength (UTS), tensile yield strength (TYS), and elongation (EL) values of 177 MPa, 139 MPa, and 4.27%, respectively. Compared to the as-cast alloy, UTS, TYS, and EL increased by 9.94%, 11.2%, and 27.1%, respectively. The study reveals that all three LPSO phases exhibit excellent dislocation hindering effects, effectively enhancing strength of the alloy. Additionally, the N-LPSO phase, due to its dense distribution, forms numerous dislocation channels within the grains, dispersing stress concentration within the grains to improve plasticity of the alloy. Furthermore, the interaction between the N-LPSO phase and the other phases in the alloy can also enhance plasticity of the alloy. Therefore, the alloy treated for 4.5 h demonstrates a synergistic improvement in strength and plasticity. Research has revealed that the precipitation mechanism of the N-LPSO phase in the as-cast WA121 alloy involves the formation of an Al-rich region adjacent to the needle-like Mg₂₄Y₅ phase. Subsequently, the Y element provided by the dissolving Mg₂₄Y₅ phase reacts with this region, ultimately leading to the formation of the needle-like LPSO phase.